1. Field of the Invention
This invention relates to methods of determining the location of an event of interest in conjunction with a satellite positioning system, such as GPS.
2. Description of Related Art
The global positioning system is a satellite-based navigation system consisting of a network of up to 32 orbiting satellites (called space vehicles, “SV”) that are in six different orbital planes. 24 satellites are required by the system design, but more satellites provide improved coverage. The satellites are constantly moving, making two complete orbits around the Earth in just under 24 hours.
The GPS signals transmitted by the satellites are of a form commonly known as Direct Sequence Spread Spectrum employing a pseudo-random code which is repeated continuously in a regular manner. The satellites broadcast several signals with different spreading codes including the Coarse/Acquisition or C/A code, which is freely available to the public, and the restricted Precise code, or P-code, usually reserved for military applications. The C/A code is a 1,023 bit long pseudo-random code broadcast with a chipping rate of 1.023 MHz, repeating every millisecond. Each satellite sends a distinct C/A code, which allows it to be uniquely identified.
A data message is modulated on top of the C/A code by each satellite and contains important information such as detailed orbital parameters of the transmitting satellite (called ephemeris), information on errors in the satellite's clock, status of the satellite (healthy or unhealthy), current date, and time. This part of the signal is essential to a GPS receiver determining an accurate position. Each satellite only transmits ephemeris and detailed clock correction parameters for itself and therefore an unaided GPS receiver must process the appropriate parts of the data message of each satellite it wants to use in a position calculation.
The data message also contains the so called almanac, which comprises less accurate information about all the other satellites and is updated less frequently. The almanac data allows a GPS receiver to estimate where each GPS satellite should be at any time throughout the day so that the receiver can choose which satellites to search for more efficiently. Each satellite transmits almanac data showing the orbital information for every satellite in the system.
A conventional GPS receiver reads the transmitted data message and saves the ephemeris, almanac and other data for continual use. This information can also be used to set (or correct) the clock within the GPS receiver.
To determine position, a GPS receiver compares the time a signal was transmitted by a satellite with the time it was received by the GPS receiver. The time difference tells the GPS receiver how far away that particular satellite is. By combining distance measurements from multiple satellites, position can be obtained by trilateration. With a minimum of three satellites, a GPS receiver can determine a latitude/longitude position (a 2D position fix). With four or more satellites, a GPS receiver can determine a 3D position which includes latitude, longitude, and altitude. The information received from the satellites can also be used to set (or correct) the clock within the GPS receiver.
By processing the apparent Doppler shifts of the signals from the satellites, a GPS receiver can also accurately provide speed and direction of travel (referred to as ‘ground speed’ and ‘ground track’).
Nearly all current GPS receivers work by processing signals from the satellites in “real time”, as they come in, reporting the position of the device at the current time. Such “conventional” GPS receivers invariably comprise:                an antenna suitable for receiving the GPS signals,        analogue RF circuitry (often called a GPS front end) designed to amplify, filter, and mix down to an intermediate frequency (IF) the desired signals so they can be passed through an appropriate analogue-to-digital (A/D) convertor at a sample rate normally of the order of a few MHz,        digital signal processing hardware that carries out the correlation process on the IF data samples generated by the ND converter, normally combined with some form of micro controller that carries out the “higher level” processing necessary to control the signal processing hardware and calculate the desired position fixes.        
The less well known concept of “Capture and Process Later” has also been investigated. This involves storing the IF data samples collected by a conventional antenna and analogue RF circuitry in some form of memory before processing them at some later time (seconds, minutes, hours or even days) and often at some other location, where processing resources are greater.
The key advantages of the Store and Process Later approach over conventional GPS receivers are that the cost and power consumption of the capturing device are kept to a minimum as no digital signal processing needs be done at the time of capture, and the captures can be very short (e.g. 100 ms). If the subsequent signal processing is done when the relevant satellite data (ephemeris etc) can be obtained via some other method, this approach also removes the need to decode the (very slow) data message from the SVs in the capturing device, which in many cases leads to unacceptably long times to start up conventional devices.
One problem with GPS systems in the case of battery-operated portable devices is that they can drain power, and thus give rise to a short battery life.
Another more general problem is that at times the GPS environment may be difficult, for example indoors or in an “urban canyon” between high rise buildings, so that it may not be possible to do a location fix using GPS. The sensitivity of GPS can be improved which can help this problem, but does not solve it completely, as there will always be situations in which there are insufficient satellite signals. Increased sensitivity also increases power consumption and cost, due to the extra computation and processing performed.
Tracking solutions provide a way forward, as a position found outdoors can then be tracked into and through difficult environments. The sensitivity can be substantially improved by this means, often giving good performance. The “last known position” can also be reported. However a tracking system consumes power consumption continually, even if actually the results are not used—the track must be maintained in case it is required later.